Phytogeography of the Pacific Coast of North America

This study was designed to explore floristic variation along the Pacific coast from Cook Inlet, Alaska (61º30’N), to the southern tip of Baja California, Mexico, at 23º02’N. Data corresponding to 965 phytosociological relevés (including 686 vascular plants) obtained by sampling 279 coastal sites were examined. Using an objective classification system (Average Linkage Clustering) and Sørensen’s index, floristic data acquired in fieldwork were related to the biogeographical regions of the study area. According to the worldwide distribution pattern of genera and species, the registered taxa were grouped into phytogeographical elements. These elements, along with the climatic classification of the sites and comparisons among the main coastal vegetation types, gave rise to a phytogeographical classification of the study area in which four regions (North American Boreal, Rocky Mountain, Californian and Xerophytic-Mexican) and eight floristic provin - ces were distinguished. Each province is characterized by its climate, changes in phytogeographic elements, the presence of endemic genera or species, and by its characteristic vegetation types. Floristic differences among provinces are related to palaeoclimatic and palaeogeographical events. The boreal and temperate provinces show high percentages of broadly distributed elements, whose origins can be found in the Boreotropical or Arctotertiary geoflora that dominated the Northern Hemisphere during the Tertiary. The floristically richest provinces are the three that occupy the Mediterranean zone, since these provinces harbour elements of the Arctotertiary and Madrotertiary geofloras. The flora of tropical Baja California is characterised by a high number of taxa related to Neotropical flora, especially to those showing links with South America.